1. Field of the Invention
The present invention relates to a door locking mechanism of the chain guard type that permits holding a door partially ajar so as not to allow a person into the premises before the chain is released.
More particularly, the invention relates to a door looking mechanism which advantageously combines the features and advantages of both a standard dead lock and a safety chain type door lock.
2. Description of the Prior Art
There are numerous door locking mechanisms of the chain guard type presently available on the market, and numerous patents dealing with improvements thereto. Those believed to be most relevant to this invention are as per Canadian Patent numbers - 710 033-, -718 626-, - 801 377-, and U.S. Pat. Nos. 3,125,875, 3,161,035, and 3,395,556.
Other inventions, such as those described in British Patent 6448 of Mar. 15, 1912 and German Patent 333931 of Mar. 5, 1921 are relevant because they show that the dead lock advantage was a desireable feature even at these early dates.
Basically, each mechanism of this type comprises a chain secured at one end to a key operated lock which is attached to the door jamb. At its other end, the chain is provided with a bolt that is inserted and slid in a keeper provided for this purpose with an elongated slot terminating in an enlarged opening at the end of the slot away from the lock. The opening serves for the insertion of the bolt. When not in use the bolt is removed from the keeper.
This type of mechanism serves its purpose well so long as the occupant is present and on guard in the premises. It is however not completely effective when the occupant is absent, even though the bolt is in the keeper and the lock member is secured in the key operated lock. In such circumstances it is possible for an unauthorized person to cause the bolt to slide toward the enlarged opening of the keeper slot and have the bolt finally fall out of the keeper. The hand is used if the unauthorized person has managed to gain entry to the premises or a hand may be inserted via a broken window in the door. Alternatively an extension tool such as a coat hanger or rubber band with thumbtack can be used from outside. It is of course possible to overcome this drawback by shortening the chain but then the mechanism is no longer useful as a safety door lock since, in normal use, the partial door opening is decreased and may no longer be sufficient to allow a person on the inside to identify persons on the outside or to allow the passage of parcels and other articles or even inconvenience access with the key from the outside unless the chain is made free by using the key.